1. Field of the Invention
The present invention relates to piezoelectrically actuated vibrating devices. More particularly the present invention relates to feedback-responsive piezoelectric vibrating devices by which vibrational energy may be introduced into heterogeneous plastic work materials to affect the physical or chemical character of the work materials.
2. Description of the Prior Art
The present invention is a unique vibrational energy producing device which may be advantageously used in a preferred embodiment of the invention to modify the texture or character of a heterogeneous plastic or slurry work material.
For illustrative purposes, the following disclosure describes an application of the preferred embodiment of the invention wherein acoustic energy generated within the device is introduced into a plastic concrete mass in order to advantageously affect physical and chemical properties of the plastic concrete mass. It will be understood, however, that similar devices constructed in accordance with the present invention may be used to advantageously modify the texture or character of many other work materials, particularly plastic or wet materials such as plaster, wet soil, coal, sewer sludge, cement and the like.
It will also become apparent from the following disclosure that similar devices constructed in accordance with the present invention may be used in many applications where it is advantageous or desirable to efficiently pass acoustic waves into work media for purposes other than modifying the character of the work material.
It is often beneficial to work a plastic material into a more desirable state before use or disposal of that material. For example, the prior art teaches that vibrational energy may be introduced into work materials to produce effects such as dewatering or consolidation of the work materials. A problem with prior methods of introducing vibrational energy into such materials is associated with the lack of control of the vibrators, per se, and the lack of control of the vibrations which they produce.
One advantageous application of introducing vibrational energy into plastic work materials, such as concrete, is to expeditiously consolidate the material in order to produce an end product having as nearly a uniform density as possible, by encouraging and assisting the upward migration of liquids (e.g. water) and gases (e.g. air) from the plastic work material which would otherwise migrate slowly or not at all. Entrapment of air and water weakens concrete and like materials, and the slow migration of the air and water would disadvantageously increase the time required to place and finish a plastic concrete mass.
Prior art methods associated with the dewatering of work materials mostly teach devices which use "dumb" vibrators. In this context, the term "dumb vibrators" means vibrators which generate vibrational energy either at random frequencies or at specific predetermined frequencies, regardless of, and non-responsive to, the local properties of the work material. With prior methods the work material (e.g. plastic concrete) typically is simply shaken at a relatively low frequency in order to cause some of the water and air to rise to the surface. Internal combustion engine vibrators are typically used in the prior art in order to generate the vibrations. These prior methods of vibrating work materials are very inefficient, and often have little or no effect on the work material.
U.S. Pat. No. 5,527,175 to Face et al teaches a method for introducing vibrational energy into plastic concrete structures at or near the natural resonant frequency of the work material (i.e. plastic concrete) to expedite the consolidation and setting of the plastic concrete. Various schemes, for example eccentrically weighted motors and magnetostrictive actuators, are proposed for generating vibrations at the resonant frequencies of the work material. Problems with eccentrically weighted motors are that they are incapable of generating vibrations at high frequencies (i.e. greater than 100 Hz), the frequency of vibrations cannot be readily varied or fine tuned, and the device itself is typically cumbersome, very heavy, and requires excessive maintenance, as is necessary when working in the resonant frequency range of many plastic work materials.
Another problem with magnetostrictive vibrators is that they are fragile. In addition, magnetostrictive vibrators tend to degrade over time. That is, over extended periods of time the vibrational characteristics of the output of a magnetostrictive device change for a given input to the device. Also, magnetostrictive vibrators are expensive, and they inherently require the generation of a magnetic field in order to operate (which may be potentially damaging in some applications).
U.S. Pat. No. 5,527,175 teaches a method for determining the frequency of vibrational energy to be imparted into the work material corresponding to the natural resonant frequency of the work material. U.S. Pat. No. 5,527,175 teaches providing sensors which are in electrical communication with a processor unit. Based upon data provided by the sensors about how much power is being expended to run the vibrators, the processor unit selects a preferred frequency corresponding to that frequency at which the least amount of power is required to vibrate the work material.
A problem associated with selecting the vibrational frequency using the method taught in U.S. Pat. No. 5,527,175 is that the processor unit is inherently affected not only by the natural frequency of the concrete, but also the natural resonant frequency of the apparatus itself. Thus the frequency of vibrations which is typically selected using the method taught in the U.S. Pat. No. 5,527,175 inherently corresponds to that frequency at which the least amount of power is required to vibrate both the concrete work mass and the vibrating apparatus itself. It will be appreciated that in certain instances the natural resonant frequency of the work material per se will not be the same as the natural frequency of the vibrating apparatus and the work material combined.
Thus it would be desirable to provide a device that is capable of efficient application of "smart" vibrations to a work material at frequencies corresponding to a local resonant frequency of the work material and independent of the natural frequency of the vibrator apparatus itself. In this context "smart vibrations" means vibrators which generate vibrational energy at varying frequencies, wherein such frequencies are varied in a controlled fashion in response to sensed local properties of a work material.